Field of the Invention
The invention relates to a method for monitoring a feedwater supply to a continuous-flow or once-through steam generator, the steam temperature of evaporated feed water being employed for determining the feedwater supply rate in the method. The invention further relates to a device for performing the method.
Whereas, a circulated water/steam mixture evaporates only partially in a natural-circulation steam generator, the heating of vertically arranged evaporator tubes forming gas-tight containment walls of a combustion chamber in a continuous-flow or once-through steam generator leads to a complete evaporation of the flow medium in the evaporator tubes in a single pass.
Continuous-flow or once-through steam generators are conventionally operated, from an automatic control engineering standpoint, in a manner that predetermined nominal or setpoint values for the steam capacity and the outlet temperature of a superheater located downstream of the evaporator are maintained as precisely as possible. The termination or end of evaporation and, therefore, simultaneously, the commencement of steam superheating are not fixed locally. On the contrary, the end of evaporation is initiated independently, inter alia, dependent upon load and upon the condition of contamination of the heating surfaces of the evaporator or superheater as well as upon the fuel/water content. Therefore a marked difference from the natural-circulation steam generator exists, wherein the end of evaporation is fixed in a water/steam separating drum. An indicator for the water level in the drum provides the operating personnel with a measuring instrument by which a sufficient feedwater supply to the natural-circulation steam generator can be monitored.
In contrast therewith, in a continuous-flow or once-through steam generator, the monitoring of a sufficient water supply occurs, for example, via the steam temperature of the evaporated feedwater, a practice which has become known, for example, from the published German Patent document DE 32 43 578 C2. Monitoring may also be effected indirectly, however, in particular, by observing an injected flow of water in the superheater as a measurement value. On the one hand, however, this measurement is not precise because such injected flows of water vary due to operating conditions. On the other hand, a regulating or control action pursuant to a regulation based upon this measurement value is considerably delayed disadvantageously.